In electrophotography, an electrophotographic photoreceptor comprising a support which is coated with a photoconductive material in which the electric resistance varies in accordance with the amount of the exposure which is imparted thereto during image exposure is employed, as described in Carlson's U.S. Pat. No. 2,297,691. The photoconductive material is, in general, charged with a uniform surface electric charge in a dark place, after dark adaptation for an appropriate period of time. Next, this material is imagewise exposed with an irradiation pattern whereby the surface electric charge is reduced in accordance with the relative energy contained in the respective parts of the irradiation pattern. The surface electric charge or the electrostatic latent image which remains on the surface of the photoconductive layer (electrophotographic light-sensitive layer) is thereafter brought into contact with an appropriate electroscopic displaying substance or a toner to form a visible image.
The toner is contained in an insulating solution or in a dry carrier, and in both cases, the toner may be applied on the surface of an electrophotographic light-sensitive layer in accordance with the electric charge pattern. The displaying substance thus applied on the surface may be fixed thereon by heat, pressure, solvent vapor or the like known means. The electrostatic latent image may be transferred to a second support (such as paper, film or the like). In the same manner, the electrostatic latent image which is transferred to the second support may be developed thereon.
In electrophotography, fundamental characteristics which are required in the electrophotographic photoreceptors are that (1) the photoreceptor may be charged to an appropriate potential in a dark place, (2) the electric charge is hardly lost in the dark place, and (3) the electric charge may rapidly be erased by light irradiation.
Photoconductive materials which have heretofore been used in electrophotographic photoreceptors are selenium, cadmium sulfide and zinc oxide.
Although these inorganic substances have various merits, they have, in fact, various defects. For example, selenium which until now has mainly been used, sufficiently satisfies the aforesaid conditions (1) through (3), but has various defects in that it is complicated to manufacture, its manufacturing cost is high, it is lacking in flexibility, it can hardly be formed into a belt-like article, it is sensitive to heat and mechanical shock and special attention is required for the handling thereof. Cadmium sulfide or zinc oxide, when used as the photoconductive material in an electrophotographic photoreceptor, is dispersed in a binder of a resin and thus can be easily manufactured, but both of these materials are mechanically defective with respect to smoothness, hardness, tensile strength and abrasion resistance. Therefore, cadmium sulfide and zinc oxide are scarcely fit for repeated use.
In order to eliminate the defects of these inorganic substances, electrophotographic photoreceptors of various organic substances have been proposed and some have actually been used. For example, there are electrophotographic photoreceptors comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237); poly-N-vinylcarbazole as sensitized with a pyrylium salt type dye (Japanese Patent Publication No. 25658/73); electrophotographic photoreceptors mainly comprising an organic pigment (Japanese Patent Application (OPI) No. 37543/72) (the term "OPI" as used herein refers to a "published unexamined Japanese patent appication"); and electrophotographic photoreceptors mainly comprising an eutectic complex of a dye and a resin (Japanese Patent Application (OPI) No. 10785/72).
These electrophotographic photoreceptors of organic substances may easily be manufactured by suitably selecting the binder to be used together with the organic substance and coating the mixture on a proper support, and therefore, the productivity is extremely high and inexpensive photoreceptors may be provided. Further, the mechanical characteristics and the flexibility may be improved, and the light-sensitive wavelength may freely be controlled by selecting the dye and the organic pigment to be used. On the contrary, however, the light sensitivity is low and the photoreceptors are scarcely fit for repeated use, and therefore, they do not sufficiently satisfy the necessary characteristics.
In another area of technology, a recording method has been developed where a beam of high energy density is irradiated onto an information-recording medium to vary the physical constants thereof such as percent transmission, reflectivity or refractive index for the purpose of recording the necessary information in the medium. This recording method has numerous advantages in that a contrasting image of an extremely high resolving power may be formed, addition of further information is possible, and simultaneous exposure and recording are possible. The method is suitable for recording an output of an electronic computer or a time system signal is transferred, and therefore, the method is applied to COM (computer output micro), microfacsimile, printing original, photo-disc, etc.
For instance, the recording medium to be used in photo-disc technology contains optically detectable small pits of about 1 micron or so in a spiral form or a circular track form, and information of high density may be recorded therein. In the writing of the information in the disc of this kind, a laser which is focused on the surface of the laser-sensitive layer is scanned to form pits only on the portions of the surface thereof which are irradiated with the laser ray, whereupon the pits are formed in a spiral form or a circular track form. In a heat mode-recording system, the laser-sensitive layer absorbs heat energy by the laser beam irradiation, whereupon small pits are formed in the laser-irradiated parts by evaporation or fusion of the layer.
The information which is recorded in the photodisc in the manner as described above is detected by scanning the laser along the track and reading the optical variation between the parts where the pits are formed and the other parts where the pits are not formed.
Conventional information-recording media for heat mode-recording, which have heretofore been used, comprise a recording layer of a thin film made of metal and/or metal oxide semi-metal dielectrics or of a thin film containing a self-oxidizable binder and a dye. The recording layer is provided on a plastic or the like transparent support, and a protective layer is coated on the recording layer.
However, thin films comprised mainly of an inorganic substance, which have been used in conventional recording media, have high reflectivity to the laser ray and, therefore, have a number of problems in that the laser utilization efficiency is low and a high sensitivity characteristic cannot be attained or the output of the laser ray in the recording is extremely increased.
On the other hand, organic compounds become unstable with a shift of the absorption characteristic to the long wavelength range and they often decompose with a slight increase in temperature. Thus, these compounds have various problems when used in thin films for recording layers.
The information-recording media must have various characteristics which are required for DRAW (direct read and write), such as a high absorption efficiency to the laser ray which is used, a sufficient reflectivity for the focal control in the information reading, and a high stability of the recorded image. However, recording media which have an organic thin film and which are satisfactory for practical use have not as yet been developed up to the present.